1. Prior Art
This invention is directed to piezoelectric actuators and in particularly to for actuating control valves useful or injection valves in motor vehicles of the type, having an actuator body in the form of a multilayered laminate made up of stacked layers of piezoelectric material with intervening metallic or electrically conductive layers that function as electrodes, which, in alternating fashion, electrically contact common electrode strips disposed on the circumference, in the longitudinal direction of the actuator body, and having electrical connecting conductors which, in order to supply the operating voltage to the piezoelectric actuator, electrically contact the common electrode strips.
2. Description of the Prior Art
A piezoelectric actuator of the type with which this invention is concerned has been disclosed, for example, by DE 196 50 900 A1 assigned to Robert Bosch GmbH.
If a pulsating electrical voltage acts on their electrode layers, multilayered piezoelectric actuators of this kind execute analogously pulsating strokes by changing the distance between their two end faces. The end face oriented toward the control or injection valve tappet-like mechanism, which transmits the actuator stroke to the valve.
In the above-mentioned DE 196 50 900 A1, the connection of the piezoelectric actuator is produced in the active region of the actuator body. As a result, the electrode strips, which are disposed on both sides of the actuator body and transmit the electrical voltage to the electrode layers, overlap the full length of the active region of the actuator body and this results in the disadvantage that the electrical connecting conductors must execute the stroke along with the actuator. In addition, the contacting of this actuator with the common electrode strips is problematic since such a contact that is produced by a solder connection has a tendency to develop fractures during operation of the actuator.
A piezoelectric actuator of this generic type should be embodied so that a reliable contacting of the connecting conductors with the common electrode strips is achieved over the life of the piezoelectric actuator or the valve that is equipped with it and that at the same time, the usually soldered connections can be replaced by clamped connections.
The essence of the invention lies in taking the contacting of the connecting conductors with the common electrode strips out of the active region of the piezoelectric actuator.
This is advantageously achieved in accordance with the invention by virtue of the fact that on at least one end face remote from the valve end, the actuator body has a covering layer made of piezoelectrically inactive ceramic material and that the connecting conductors electrically contact the common electrode strips only in the vicinity of this covering layer.
Although for the contacting of the piezoelectric actuator of this invention, the only essential element is the piezoelectrically inactive covering layer disposed on the end face of the actuator body remote from the valve end, the piezoelectric actuator body can naturally also have a piezoelectrically inactive covering layer on its end face oriented toward the valve.
Because the contacting of the connecting conductors with the common electrode strips is disposed in the vicinity of the piezoelectrically inactive covering layer of the actuator body, which is not subjected to the actuator stroke, there are no longer any connections in the active region of the piezoelectric actuator and the development of fractures at solder points and the occurrence of contacting problems are prevented during its operation.
In an exemplary embodiment of the piezoelectric actuator according to the invention, the common electrode strips contact the connecting conductors on the circumference of the covering layer.
In another exemplary embodiment of the piezoelectric actuator according to the invention, the common electrode strips are routed to the end face of the covering layer and contact the connecting conductors there.
The contacting of the common electrode strips with the connecting conductors can either be by soldering or clamping.
In order to route the contacting conductors to the actuator body through a valve housing, it can be advantageous if the contact points where the connecting conductors contact the common electrode strips on the piezoelectrically inactive covering layer are disposed diametrically opposite from one another. This is true both for piezoelectric actuators with a circular cross section and for those with a rectangular or square cross section.
It should be noted that the embodiment of the common electrode strips along the actuator body is not a subject of this invention so that in the exemplary embodiments described below, it is simply assumed that these electrode strips have the form of flat bands or ribs resting against the circumference of the actuator body.